Glass that is fairly clear in color and highly transmissive to visible light (e.g., at least 75% transmissive, or even more preferably at least 80% transmissive) is sometimes desirable. One way of achieving such as glass is to use very pure base glass materials (e.g., substantially free of colorants such as iron). However, base materials with a high degree of purity are expensive and thus not always desirable and/or convenient. In other words, for example, the removal of iron from glass raw materials has certain practical and/or economical limits.
As can be appreciated from the above, glass raw materials (e.g., silica, soda ash, dolomite, and/or limestone) typically include certain impurities such as iron. The total amount of iron present is expressed herein in terms of Fe2O3 in accordance with standard practice. However, typically, not all iron is in the form of Fe2O3. Instead, iron is usually present in both the ferrous state (Fe2+; expressed herein as FeO, even though all ferrous state iron in the glass may not be in the form of FeO) and the ferric state (Fe3+). Iron in the ferrous state (Fe2+; FeO) is a blue-green colorant, while iron in the ferric state (Fe3+) is a yellow-green colorant. The blue-green colorant of ferrous iron (Fe2+; FeO) is of particular concern when seeking to achieve a fairly clear or neutral colored glass, since as a strong colorant it introduces significant color into the glass. While iron in the ferric state (Fe3+) is also a colorant, it is of less concern when seeking to achieve a glass fairly clear in color since iron in the ferric state tends to be weaker as a colorant than its ferrous state counterpart.
In view of the above, it is apparent that there exists a need in the art for a new glass composition which allows a glass to have fairly clear color and/or high visible transmission, without having to resort to extremely pure (i.e., free of iron) glass raw materials.
A known clear glass is PPG's Starphire glass; see the composition thereof set forth in FIG. 1. Unfortunately, PPG Starphire glass (at 6 mm thick) can only achieve an a* color value of −0.29 and a b* value of 0.11. This color is not neutral enough for certain applications. In other words, a* and/or b* value(s) closer to zero are desired in certain applications.
Another known clear glass is set forth as “Regular clear” glass in FIG. 1. Again, unfortunately, this glass can only achieve an a* color value of −1.54 and a b* value of 0.37. This color is not neutral enough for certain applications. In other words, a* and/or b* value(s) closer to zero are desired in certain applications.
U.S. Pat. No. 6,949,484 (commonly owned and hereby incorporated herein by reference) discloses another clear glass composition. However, the examples set forth in the '484 patent cannot achieve a combination of high visible transmission (e.g., at least about 90%) and very neutral a* and b* color values. Moreover, large amounts expensive cerium oxide are required in certain examples of the '484 patent.
U.S. Pat. No. 6,716,780 (commonly owned and hereby incorporated herein by reference) discloses a grey glass composition. Unfortunately, the examples set forth in the '780 patent cannot achieve a combination of high visible transmission (e.g., at least about 90%) and very neutral a* and b* color values.
In view of the above, it will be apparent that there exists a need in the art for a clear glass composition capable of realizing a combination of high visible transmission (e.g., at least about 90%) and very neutral a* and b* color values so as to provide very clear color to viewers and the like if desired.